1. Field of the Invention
The invention relates to a molded sheet with a core made from a number of core layers and, located on one or both sides of the core, substrate layers, decorative layers, and clear layers, where flame retardants have been added to the molding, and to a use of the molding, and to a process for its production.
2. Description of the Related Art
For the purposes of the present invention, xe2x80x9ca molded sheetxe2x80x9d means either flat sheets or else moldings which have been produced in a mold and are not flat, these having thickened face edges, for example, or a profile with different thicknesses along the length of the format.
Sheets of this type are generally composed of a number of thermoset-binder-impregnated supportive plies, which are molded in a press using increased pressure and temperature to give a sheet or a molding, by virtue of curing of the binder. These sheets and/or moldings are used either for the fitting-out of interiors or else for external applications, and have to comply with certain fire standards. For the fitting-out of interiors, the moldings are used in the office, kitchen, bathroom, laboratory equipment, furniture, etc., and an example of their use in an external application is as cladding. In order to comply with the fire standards, it is usual in the prior art to add flame-retardant additives, known as flame retardants, to the moldings.
Moldings of sheet type usually have a layer-like structure and are composed of a substrate core which has two or more layers and has been laminated, on one side or on each side, to a substrate layer, a decorative layer, and a clear layer. The decorative layers are generally papers which comprise xcex1-cellulose and which have been colored, pigmented, printed on, or provided with a decorative effect, and which have been converted into a decorative premolding by virtue of saturation and drying, using a low-molecular-weight melamine resin. Each individual decorative layer may comprise a second layer which has been applied to the outer side of the decorative layer and is produced from a paper which comprises xcex1-cellulose and has been impregnated with a low-molecular-weight melamine resin but has not been printed and comprises no decorative element. This is what is known as a clear layer, which has the properties of the premolding. It is usual for there to be no flame retardants present in the decorative layers and clear layers. The core is composed of a number of layers, generally composed of a paper which comprises xcex1-cellulose (kraft paper), and which are laminated to one another to give the core, by virtue of saturation with a low-molecular-weight phenolic resin followed by drying. This layer has usually been treated with a flame retardant.
EP-B 0 081 147 describes a decorative panel suitable for external applications and composed of a pressed core made from fibers surrounded by heat-cured phenol-formaldehyde resin, and which has a decorative layer on one or both surfaces of the core. The core is composed of wood fibers and/or cellulose fibers, which have been coated with a heat-curable phenol-formaldehyde resin in aqueous solution or dispersion. The method of producing one embodiment of this panel is that one or more mutually superimposed, mechanically precompacted layers based on wood particles, impregnated with the heat-curable phenol-formaldehyde resin, are laminated with heat to form the core of the panel, thus curing the synthetic resin.
Hot-laminated decorative high-pressure laminates (HPLs) of this type are what are known as compact moldings, the structure of which is either identical with or similar to that of compact sheets in accordance with EN 438-1, ISO 4586, or DIN 16926.
WO 99/45061 describes composite materials which are composed of a fibrous and/or woven material, impregnated and hardened with a resin matrix. The resin matrix based on an epoxide/anhydride reactive resin has been rendered flame-retardant using reactively incorporated phosphorus compounds based on acid derivatives. The flame-retardant composite material also comprises fillers, defoamers, flow auxiliaries, adhesion promoters, and reaction accelerators commonly used in epoxy resin chemistry, e.g. tertiary amine and/or imidazole, and organometallic complexes. The phosphorus content, based on the resin matrix, is from 0.5 to 5% by weight. Composite materials of this type are used as lightweight materials in vehicle construction, for rail vehicles, bodywork of motor vehicles, and components of ships or of aircraft.
It is also known that flame-retardant additives can be used in plastics. For example, WO 96/09344 describes a flame-retardant, glass-fiber-reinforced polyamide resin composition which comprises from 10 to 40% by weight of melamine phosphate, melamine pyrophosphate, in particular dimelamine pyrophosphate or melamine polyphosphate, or else a mixture of these. Other preferred halogen-free flame retardants which may be present in the polyamide resin composition are zinc borate, zinc phosphate, melamine sulfate, and ammonium polyphosphate. Conventional additives, such as lubricants, dispersing agents and adhesion promoters, may also be added, examples of these being stearates, phosphonates, fatty acid amides, and Aerosils. Glass-fiber-reinforced polyamide resin compositions of this type are particularly suitable for producing moldings which are used in the electrical or electronics industry.
The cores of known sheets comprise resins selected from the group consisting of amino plastics, epoxy plastics, urethane plastics, thermosets, and mixtures of these. Flame retardants are generally added to these resins. Flame retardants used are phosphorus-containing additives or additives with phosphorus-nitrogen. Other known flame retardants are borates or hydroxides of aluminum, zinc or magnesium, and halogen-containing organic compounds. If the core is composed of a prefabricated premolding made from wood fibers and/or from cellulose fibers, the flame retardant may be added to the binder for the wood fibers or cellulose fibers. Besides this, there is also the possibility of adding the flame retardant directly to the wood fibers or wood flour used to mold the premoldings. In the known sheets which comprise a flame retardant there is always uniform distribution of the flame retardant, both in the core layer and in the decorative layers, in the event that these have also been provided with a flame retardant. Decorative layers of this type are composed, for example, of a crosslinked acrylic, urethane, epoxy, or melamine resin which has been modified using acrylic resin and comprises fillers and/or colorants. In each case, a flame retardant is then added to these resins.
In the event that a molding is heated to the ignition temperature, the decorative layer first begins to burn, followed by the core layers situated thereunder, until the entire molding burns. To prevent fire it is important that the moldings do not ignite in the event of a fire, since the ignition of the moldings accelerates the heating process, therefore generating more heat which again accelerates the heating process. This upward spiral of phenomena has a very substantial effect on the fire performance of the moldings and their classification under standards.
It is an object of the invention to improve the flame retardancy of moldings of the type described at the outset in such a way as to increase the effectiveness of the flame retardants present in the moldings. This object is achieved in that the distribution of the flame retardants is non-uniform across the thickness of the molding.
The invention provides a sheet molding with a core made from a number of core layers, and, located on one or both sides of the core, substrate layers, decorative layers, and clear layers, where flame retardants have been added to the molding, wherein the distribution of the flame retardants across the thickness of the molding is non-uniform.
The invention also provides a process for producing a sheet molding which comprises inserting a release layer, a clear layer, a decorative layer and/or a substrate layer, where the decorative layer is modified with an amount m1 of a flame retardant and the substrate layer is modified with an amount m2xe2x89xa6m1 of a flame retardant, and a prefabricated core composed of a number of core layers, where each core layer has been modified with a similarly large amount m3 of a flame retardant, where m3xe2x89xa6m2xe2x89xa6m1, and inserting another substrate layer and/or a decorative layer, each of which has been provided with a flame retardant, and a clear layer, and a release layer, into an open press, and hot pressing, under pressure, all of the layers present in the press, whereupon the prefabricated layers are cured, the resins of the layers liquefy, and the layers are molded to give a massive homogeneous molding.
The invention further provides a sheet molding which comprises:
a) at least one core layer, which core has a front surface and a rear surface;
b) a front substrate layer attached to the front surface of the core, and a rear substrate layer attached to the rear surface of the core;
c) a front decorative layer attached to the front substrate layer, and a rear decorative layer attached to the rear substrate layer; and
d) a front clear layer attached to the decorative layer, and a rear clear layer attached to the rear decorative layer;
wherein at least one flame retardant material is present in at least one of the layers, and wherein the distribution of the flame retardant across the thickness of the molding is non-uniform.
The invention still further provides a process for producing a sheet molding which process comprises:
a) providing an open heat press;
b) inserting a plurality of pre-molding layers into the open press, which pre-molding layers comprise, in order, a front release layer, a front clear layer, a front decorative layer, a front substrate layer, a prefabricated core comprising at least one core layer, a rear substrate layer, a rear decorative layer, a rear clear layer, and a rear release layer, wherein the front decorative layer comprises an amount m1 of a flame retardant, the front substrate layer comprises an amount m2 of a flame retardant, wherein m2xe2x89xa6m1, and wherein each core layer comprises an amount m3 of a flame retardant, wherein m3xe2x89xa6m2xe2x89xa6m1, and wherein the rear substrate layer and the rear decorative layer each comprises a flame retardant material;
b) hot pressing, under pressure, all of the pre-molding layers present in the press, such that the prefabricated core becomes cured, any resins of the pre-molding layers liquefy, and the pre-molding layers are molded to the core to thereby form a sheet molding.